U.S. patent application number 16/152784 was filed with the patent office on 2019-04-11 for method for correcting the critical dimension uniformity of a photomask for semiconductor lithography.
The applicant listed for this patent is Carl Zeiss SMS Ltd., Carl Zeiss SMT GmbH. Invention is credited to Ute Buttgereit, Vladimir Dmitriev, Kujan Gorhad, Yuval Perets, Thomas Scheruebl, Thomas Thaler, Joachim Welte.
Application Number | 20190107783 16/152784 |
Document ID | / |
Family ID | 64666072 |
Filed Date | 2019-04-11 |
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United States Patent
Application |
20190107783 |
Kind Code |
A1 |
Thaler; Thomas ; et
al. |
April 11, 2019 |
Method for Correcting the Critical Dimension Uniformity of a
Photomask for Semiconductor Lithography
Abstract
The invention relates to a method for correcting the critical
dimension uniformity of a photomask for semiconductor lithography,
comprising the following steps: determining a transfer coefficient
as a calibration parameter, correcting the photomask by writing
pixel fields, verifying the photomask corrected thus, wherein a
transfer coefficient is used for verifying the corrected photomask,
said transfer coefficient being obtained from a measured scattering
function of pixel fields.
Inventors: |
Thaler; Thomas; (Jena,
DE) ; Welte; Joachim; (Darmstadt, DE) ;
Gorhad; Kujan; (Kfar Kama, IL) ; Dmitriev;
Vladimir; (Tzurit, IL) ; Buttgereit; Ute;
(Jena, DE) ; Scheruebl; Thomas; (Jena, DE)
; Perets; Yuval; (Moshav Beit Shearim, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carl Zeiss SMT GmbH
Carl Zeiss SMS Ltd. |
Oberkochen
Misgav |
|
DE
IL |
|
|
Family ID: |
64666072 |
Appl. No.: |
16/152784 |
Filed: |
October 5, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/70191 20130101;
G03F 1/72 20130101; G03F 1/84 20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 5, 2017 |
DE |
102017123114.5 |
Claims
1. A method for correcting the critical dimension uniformity (CDU)
of a photomask for semiconductor lithography, the method comprising
the following steps: determining a CDU to be corrected by way of a
scanner-equivalent CDU measurement, determining a transfer
coefficient as a calibration parameter, correcting the photomask by
writing pixel fields, verifying the photomask corrected thus,
wherein the transfer coefficient is used for verifying the
corrected photomask, said transfer coefficient being obtained from
a scattering function of pixel fields measured in advance.
2. The method according to claim 1, wherein the scattering function
is integrated within the respectively applicable integration limits
for a mask metrology device and for a scanner for the purposes of
determining the transfer coefficient.
3. The method according to claim 2, wherein forming a quotient from
the respective results of the integration is undertaken for the
purposes of determining the transfer coefficient.
4. The method according to claim 1, wherein the transfer
coefficient and an evaluation of a diffraction pattern caused by
regular structures on the photomask are used for the purposes of
verifying the photomask.
5. The method according to claim 4, wherein the evaluation
comprises a comparison of the absolute intensities of diffraction
maximums before and after writing the pixel fields.
6. The method according to claim 1, wherein the different
illumination schemes for a mask metrology device and a scanner are
taken into account when determining the transfer coefficient.
7. The method according to claim 2, wherein the transfer
coefficient and an evaluation of a diffraction pattern caused by
regular structures on the photomask are used for the purposes of
verifying the photomask.
8. The method according to claim 3, wherein the transfer
coefficient and an evaluation of a diffraction pattern caused by
regular structures on the photomask are used for the purposes of
verifying the photomask.
9. The method according to claim 2, wherein the different
illumination schemes for a mask metrology device and a scanner are
taken into account when determining the transfer coefficient.
10. The method according to claim 3, wherein the different
illumination schemes for a mask metrology device and a scanner are
taken into account when determining the transfer coefficient.
11. The method according to claim 4, wherein the different
illumination schemes for a mask metrology device and a scanner are
taken into account when determining the transfer coefficient.
12. The method according to claim 5, wherein the different
illumination schemes for a mask metrology device and a scanner are
taken into account when determining the transfer coefficient.
13. The method according to claim 1, comprising upon determining
that the photomask has not been corrected according to
specification, further correcting the photomask by writing a
different set of pixel fields.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn. 119
from German Application DE 10 2017 123 114.5, filed on Oct. 5,
2017, the entire content of which is incorporated herein by
reference in its entirety.
TECHNICAL FIELD
[0002] This description relates to correction of critical dimension
uniformity of a photomask for semiconductor lithography.
BACKGROUND
[0003] Electrical components consist of a plurality of structured
layers, which are created individually and in succession until the
component is completed. Each layer is transferred onto a
semiconductor substrate, the so-called wafer, by photolithography,
with a so-called mask serving as a template in each case. In
general, a mask comprises a transparent carrier material, for
example quartz glass, and a non-transparent material, generally
referred to as an absorber. This absorber is structured such that
it produces bright and dark regions on the wafer when the mask is
imaged. A photosensitive layer on the wafer, the so-called resist,
reacts with the incident light, as a result of which the resist is
structured in accordance with the mask template. Hence, the desired
structures ultimately arise on the wafer.
[0004] A widespread problem when structuring the absorber on the
mask consists of producing the structures exactly according to the
dimensional specifications. As a rule, real masks have a certain
variance, for example in a line width which, according to the
prescriptions, should be constant. Here, the so-called CDU
(critical dimension uniformity) is a measure for the line width
variance. This measure decisively determines the quality of the
mask. If the CDU of a mask exceeds a certain measure, the mask
counts as non-usable and hence as a reject because the uniformity
of the line widths on the wafer plays a decisive role for a high
yield of functioning electrical components. What makes matters even
more difficult here is that, as a result of the lithographic method
in the scanner, the line width variance produced on the wafer by
the mask is higher than the line variance of the absorber on the
mask by a factor, the so-called mask error enhancement factor
(MEEF).
[0005] Although etching methods can be used to modify the
structures on the mask to a certain extent within spatially tightly
delimited regions, the problem of a larger-area modification
regularly arises. Therefore, according to the prior art, the
so-called CDC (critical dimension control) tool is used for such
modifications. Local scattering centers, so-called pixels, or whole
regions with pixels, i.e., changes in the material structure of the
mask, are written by use of the CDC tool using a femtosecond laser.
Usually, regions provided with pixels with diameters in the region
of several millimeters or centimeters should be created. Since the
pixels are written into the quartz glass and consequently found in
the optical path upstream of the absorber, the incident light is
scattered at these pixels during the exposure process, as a result
of which some of the light no longer reaches the absorber of the
mask. Consequently, the intensity of the light reaching the
absorber is influenced by varying the pixel density. The intensity
changes triggered hereby in turn cause a line width change on the
wafer during the exposure process. Should the intensity with which
the absorber is exposed now be modulated using this technique in
accordance with the known CDU of the mask, it is possible to
compensate the line variance of the mask for the imaging in the
scanner. Expressed differently, by writing the pixels, the line
width that physically deviates on the mask is corrected on the
image that is created on the wafer. However, the absorber on the
mask is not physically modified in the process; instead, all that
changes is the imaging thereof when exposing the wafer in the
scanner.
[0006] Usually, the CDU is measured during the production process
of the mask. This is brought about, inter alia, with the aid of a
mask metrology device, i.e., an optical apparatus that emulates the
most important optical properties of a scanner and therefore
inherently captures some of the effects contributing to the
aforementioned MEEF. Examples of these apparatuses include the
wafer level critical dimension measuring appliance, abbreviated
WLCD, and the aerial image measurement system, abbreviated
AIMS.TM., with the first being used in dedicated fashion for
measuring the CDU.
SUMMARY
[0007] Certain technical differences between the WLCD and the
scanner lead to the pixels introduced by the CDC tool having
different optical effects in the two apparatuses. In order to be
able to take account of these differences in WLCD measurements, a
calibration on the basis of wafer data is necessary. However,
obtaining wafer data requires much outlay and time, which is why it
is advantageous to undertake this calibration in a different
way.
[0008] In a general aspect, the present invention specifies an
efficient method for calibrating the difference in the optical
effect of the pixels between WLCD and scanner.
[0009] The object of the invention is achieved by a device having
the features of independent claim 1. The dependent claims concern
advantageous developments and variants of the invention.
[0010] The method according to the invention for correcting the
critical dimension uniformity of a photomask for semiconductor
lithography comprises the following steps:
[0011] determining the CDU to be corrected by way of a
scanner-equivalent CDU measurement,
[0012] determining the transfer coefficient as a calibration
parameter,
[0013] correcting the mask by writing pixel fields,
[0014] verifying the photomask corrected thus,
[0015] wherein a transfer coefficient which is obtained from a
scattering function of pixel fields measured in advance is used for
verification purposes, i.e., for checking the effects of writing
the pixel field into the mask corrected thus.
[0016] Implementations of the invention can include one or more of
the following features. Here, for the purposes of determining the
transfer coefficient, the scattering function can be integrated
within the respectively applicable integration limits, in
particular over scattering angle and angle of incidence for a WLCD
and over scattering angle and angle of incidence for a scanner.
[0017] Here, the scattering function is dependent, inter alia, on
the device used to write the pixel fields and the operating
parameters of said device; i.e., it can vary by all means for
different devices and different tools.
[0018] Furthermore, forming a quotient from the respective results
of the integration can be undertaken for the purposes of
determining the transfer coefficient.
[0019] For verification of the photomask, the transfer coefficient
and an evaluation of a diffraction pattern caused by regular
structures on the mask can be used in an advantageous variant of
the invention; here, the evaluation may comprise a comparison of
the absolute intensities of diffraction maximums before and after
writing the pixel fields.
[0020] The method according to the invention makes use of the
already available AIMS (aerial image management system) and WLCD
(wafer level critical dimension) systems to optically measure the
mask, in particular to evaluate the CDU and the distribution
thereof (CDU map) over a mask. This system uses the same
illumination conditions as the scanner in respect of illumination
schemes in the pupil, wavelength, numerical aperture (NA), etc.
However, an aspect during the exposure not emulated by the system
is the simultaneously exposed area of the mask during the
measurement. The reason for the different optical effect of the
pixels in the WLCD and in the scanner can also be found here.
However, no pixels are present yet in the mask at this point.
[0021] In the next step, the CDU, which is measured by the WLCD and
which is to be corrected, is converted into an attenuation
distribution over the mask, which is to be generated by the CDC
tool, said attenuation distribution over the mask then being used
in the subsequent CDC process for the purposes of modifying the
mask. Subsequently, a verification measurement is undertaken by use
of the WLCD process in order to complete the closed loop. At this
stage, the different illumination field dimensions specified at the
outset play a role.
[0022] The illumination field in the scanner is greater than in the
WLCD. As a result, a larger area around a measurement position is
exposed at the same time, as result of which pixels at positions on
the mask that are situated further away from the measurement
position are also exposed. Stray light under different angles is
produced at each exposed pixel. Consequently, even a distant pixel
can produce stray light, which, in turn, is incident at the actual
measurement position and consequently produces an intensity
contribution at the measurement position.
[0023] However, as a result of the smaller illumination field,
pixels situated further away from the measurement position are not
exposed in the WLCD, as a result of which said pixels cannot
produce stray light which would make an intensity contribution to
the measurement position in the scanner. For this reason, a certain
pixel density in the WLCD brings about higher light attenuation
than the same pixel density in the scanner. The approach of
capturing this difference and taking it into account for WLCD
measurements consists of calibrating the ratio of the
appliance-specific light attenuation. This ratio is also referred
to as the transfer coefficient. Previously, this was only possible
by way of a comparison of the scanner results with the WLCD
results.
[0024] The novel solution approach to this end now consists of
measuring the angle-dependent scattering behaviour of the pixel
fields in advance, which can be implemented, in particular, by use
of an ellipsometer, e.g., by use of a Woollam ellipsometer. This
measurement supplies a scattering function which completely
describes the angle-dependent scattering behaviour of a pixel field
for different angles of incidence. The obtained scattering function
is also referred to as a kernel. It describes what stray light
intensities are produced under what scattering angles if a light
beam with a known angle of incidence is incident on a pixel field
with a known pixel density.
[0025] For example, a laser beam having an intensity I.sub.in is
directed toward a substrate having a pixel field, and after passing
the substrate the beam intensity I.sub.out is measured The kernel K
depends on the entering angle .alpha. and the exit angle .phi.,
represented by I.sub.out=K(.alpha., .phi.).times.I.sub.in. For
example, the kernel K may depend on the spacing of the pixels in
the substrate. In some implementations, a mask blank with a pixel
array is prepared and measured on the ellipsometer to derive
K(.alpha., .phi.) for a wide range of .alpha. and .phi..
[0026] Consequently, the kernel is a tool-independent description
of the optical behaviour of the pixels and hence it is able to
describe the different optical effects of the pixels under
different conditions, such as, e.g., in the scanner and in the
WLCD. To this end, only the boundary conditions of, in particular,
the different illumination fields of the respective apparatus need
to be taken into account in addition to the employed illumination
scheme. The maximum stray light angle, under which stray light that
contributes to the intensity at the measurement position is
generated in the respective system, can be calculated from the
illumination field dimension. For example, using the kernel
K(.alpha., .phi.) described above, different apparatuses may have
different ranges of the entering angles .alpha. and different
ranges of exit angles .phi.. Now, the portion of the light that
still arrives at the measurement position is obtained by
integrating the stray light intensities over the kernel, which
stray light intensities arise between 0 and the maximum scattering
light angle of the tool, and by comparing this value with the
incident light. The difference in the intensity of this light and
the intensity of the light incident on the pixel area is the
intensity that is lost by scattering. The quotient of the intensity
of the lost light to the intensity of the incident light is the
light attenuation for a pixel area in the 1 0 considered system.
Accordingly, the same calculation can also be undertaken for the
respective other system, with a different maximum stray light angle
applying as a result of a different illumination area. The quotient
of the apparatus-specific attenuation values then corresponds to
the transfer coefficient.
[0027] In the verification step, the effect of the pixels on the
scanner CD should be determined by use of WLCD measurements. To
this end, the WLCD measurements are carried out in accordance with
the scanner prescriptions in respect of NA, illumination, etc. By
way of a comparison of the measurements at a point before and after
the introduction of the pixels by way of the CDC tool, it is
possible to determine the optical effect of the pixels in respect
of the attenuation for the WLCD. By way of the transfer
coefficient, the effective attenuation, which will act on the
scanner, can be calculated therefrom. By way of a corresponding
adaptation of the evaluation threshold in the image measured by the
WLCD, this allows the equivalent CD value for the scanner to be
determined.
[0028] Here, it may be sufficient to measure the scattering
characteristics of the pixels for a CDC tool. Under the assumption
that the CDC tools, among themselves, produce comparable pixels and
the pixels produced by a CDC tool remain stable over the service
life of a tool, there can then be an estimate of the scattering
behaviour for a multiplicity of cases of application. It is
likewise conceivable that there can also be tool-specific and
time-limited kernels, which would then have to be taken into
account during further use of the kernel.
[0029] In parallel with the aforementioned measurement method in
the mask metrology device, such as, e.g., in the AIMS and WLCD, use
is made of a further measurement method, the so-called CDrun
method, which can be applied in the two aforementioned apparatuses.
In contrast to the aforementioned methods, in which the intensity
distribution generated by the mask is measured directly, this
measurement method is based on capturing and evaluating the
intensity values of the orders of diffraction that are generated by
the mask. The advantage of this measurement method is that it can
be implemented quicker in comparison with the above-described
method. Using it, the CDU measurement of a mask can be carried out
at a plurality of measurement points within the same time. A
boundary condition of this method is that the structures to be
measured produce dedicated orders of diffraction that are spatially
separated in the pupil. As a rule, reference is then made to
so-called regular structures, such as, e.g., optical gratings or
contact holes arranged on a regular lattice.
[0030] Furthermore, this measurement method is based on an adapted
illumination scheme in the pupil, by use of which it likewise
differs from the measurement method specified first. As a rule, an
illumination spot that is as small as possible, a so-called
monopole, in the pupil is used to expose the mask and consequently
implement the measurements. On the imaging side, the optical design
of the mask metrology device is adapted in such a way that now the
pupil, instead of the field, is imaged on the camera of the
apparatus. If a regular structure is illuminated by the monopole,
dedicated orders of diffraction arise at the absorber of the mask.
Some of these orders of diffraction are thereupon imaged as far as
the camera of the measurement apparatus by way of the imaging.
Here, these appear as further illumination spots, the distance from
one another being determined, in particular, by the lattice
constant of the regular structure. The arising orders of
diffraction are numbered as a rule, with the zero order of
diffraction corresponding to the component of the incident light
passing through the mask without being diffracted. The position of
the zero order of diffraction in the pupil therefore corresponds to
the position of the incident light and can be uniquely determined
thereby.
[0031] The object of this measurement method, just like that of the
preceding one, is to determine the CDU of a mask. In this
measurement method, a measure to this end is the ratio of the
intensity of the first order of diffraction to that of the zero
order of diffraction. Changes in this ratio can be calibrated on CD
changes on the mask. Therefore, this measurement method can be
advantageously combined with the preceding measurement method.
Here, in a first step, all measurement positions of the mask are
measured using the CDrun method and the distribution of the ratios
of the orders of diffraction over the mask are determined. In a
second step, the measured orders of diffraction ratio region is
subdivided into a fixed number of portions and exemplary
measurements are carried out in each portion according to the
measurement method described first and the CD at the measurement
position is determined. These CD values are then correlated to the
measured orders of diffraction ratios and a regression function is
calculated. This regression function now serves to convert the
measured orders of diffraction ratios into CD values on the mask,
as a result of which the CDU distribution of the mask is now known.
The latter is then corrected by the CDC tool, as already described
above.
[0032] As a result of the modified measurement method, the
verification by use of CDC tool is implemented differently than in
the preceding measurement method. The already described difference
in the illumination field dimensions and the optical effect of the
pixels resulting therefrom continue to exist in the process.
However, there is a different effect of the difference on the CDrun
measurement than in the preceding measurement method. The
attenuation caused by the introduced pixels has a homogeneous
effect on the entire pupil. Therefore, all intensities in the pupil
are attenuated by the same factor. The ratio of the order of
diffraction intensities used to determine the CD at a measurement
position however remains constant prior to and after the
introduction of the pixels as a result thereof. Therefore, the
measurement method described here is insensitive in relation to the
pixels and cannot be used in this form to verify the CDC
process.
[0033] The solution approach to this end now consists of
calculating the attenuation effective in the WLCD from the CDrun
measurements before and after the CDC process. To this end, the
ratio of the intensity of the zero order of diffraction of the
measurement at one point before the CDC process to said intensity
at the same point after the CDC process should be formed. The ratio
corresponds to the attenuation in the AIMS or WLCD. By way of the
transfer coefficient, which is determined in a manner similar to
the aforementioned method but with the additional consideration of
the different illumination schemes, it is now possible to calculate
the effective attenuation in the scanner. From the aforementioned
calibration measurement, a field measurement is used in prototype
fashion for the next step. Here, it is irrelevant whether the
prototype was recorded at the same point or at a different position
on the mask. Now, in said prototype, the intensity value at which
the average CD of the structures in the measurement correspond to
the CD value which was measured in the CDrun measurement prior to
the CDC process at the point to be verified is determined. This
intensity value can now be accordingly adapted by way of the
previously calculated effective attenuation in the scanner. Then,
the CD analysis should be repeated with this adapted intensity
value. The CD value obtained therefrom now corresponds to the CD
value in the scanner after the CDC process. The just-described
steps, specifically calculating the effective attenuation from the
CDrun measurements, calculating the effective attenuation of the
scanner by way of the transfer coefficient, finding the intensity
in a prototype measurement, adapting the intensity according to the
effective attenuation in the scanner and implementing a final CD
evaluation using the adapted intensity value, can now be repeated
for each measurement position, as a result of which a CDU map
arises, said CDU map corresponding to the CDU map in the scanner
during the exposure of the mask treated by the CDC. Consequently,
the verification of the CDC process can also be implemented in the
case of CDrun measurements.
[0034] In some implementations, after the CD uniformity of a
photomask is verified, if the CD uniformity has not been
sufficiently corrected by the pixels according to specification,
the photomask can be further corrected or set aside and not used.
For example, the specification may specify that the CD uniformity
needs to meet certain criteria. For example, the system 100 can
write another set of pixels having different parameters (e.g.,
shape, depth in the blank, or size) to try to improve the CD
uniformity. Alternatively, the system 100 may incorporate a
different repair mechanism. For example, a tool may be used to
change the structure of the photomask by etching or deposition to
repair discreet erroneous positions on the photomask.
BRIEF DESCRIPTION OF DRAWINGS
[0035] Below, aspects of the invention and employed terms are once
again explained and illustrated in more detail on the basis of the
drawing. In the figures:
[0036] FIG. 1 shows the real illumination conditions in a scanner
for cases with and without a written pixel field,
[0037] FIG. 2 shows the different extents of an illumination field
for a scanner and WLCD,
[0038] FIG. 3 shows a regular lines-and-spaces structure on a
photomask and the diffraction image thereof,
[0039] FIG. 4 shows a plot of the ratio of the intensities of
orders of diffraction against a measured CDU,
[0040] FIG. 5 shows an elucidation of the constant ratio of the
intensity of the zero and first diffraction maximum,
[0041] FIG. 6 shows an elucidation of the absolute attenuation of
the intensities of the diffraction maximums caused by the CDC
process.
[0042] FIG. 7 shows an example system for correcting the critical
dimension uniformity of a substrate.
DETAILED DESCRIPTION
[0043] This document describes an improved system and method for
correcting the critical dimension uniformity of a substrate, such
as a photomask for lithography. The invention improves the
functionality of the system by enabling the system to properly
verify whether the CD uniformity of a substrate has been adequately
corrected. If the CD uniformity after correction does not meet a
predetermined specification, the system can further correct the CD
uniformity of the substrate in subsequent process(es). The
inventive process takes into account the different illumination
schemes for a mask metrology device and a scanner when determining
a transfer coefficient used in the verification process.
[0044] In partial figures (a) and (b), FIG. 1 elucidates the real
illumination conditions in a scanner, with a punctiform
illumination scheme 1 in the present example. The left partial
figure (a) illustrates the case in which no pixels and no other
further structures are present in a photomask 2--the entry pupil 3
of the projection lens sees the unmodified, punctiform illumination
distribution 4a.
[0045] This should be distinguished from the case illustrated in
the right partial figure (b), in which the photomask 2 has been
provided with a pixel field 5. In comparison with the case without
pixels, the entry pupil 3 of the projection lens sees an
illumination distribution 4b that has been significantly modified,
both in terms of intensity and in terms of form. Illustrated by
dashed lines are those components of the illumination light which
do not pass the entry pupil 3 and which consequently are not
available for exposing the wafer. This illumination distribution
now can be established on the basis of the kernel, established by
the measurement, as a function of the illumination setting (i.e.,
the intensity distribution of the illumination light). The modified
illumination distribution 4b caused by the pixels then is incident
on the mask structures on the mask lower side, not illustrated in
the figure, along the further path of the radiation used for
imaging. It is possible to recognize a certain amount of smearing
in the illumination distribution and a distribution of the
illumination intensity over a broader spatial region than in the
case where the mask does not have any pixels. Particularly in
conjunction with the properties of the employed photoresist on the
wafer, in particular the intensity threshold of the exposure
radiation that is required for reaction of the photoresist, the CDU
can be influenced in this manner by virtue of the surface regions
on the wafer, on which the intensities of the radiation that exceed
the intensity threshold are incident, being reduced. As a result,
this also reduces the effectively exposed line width on the wafer,
and so the CDU can be adapted in this way.
[0046] As illustrated in partial figures (a) and (b) in FIG. 2, the
scanner and WLCD differ, however, in the terms of the extent of the
illumination field, as already mentioned at the outset. As a
result, stray light arises at more distant pixels of the pixel
field 5 in the scanner, said stray light, in turn, being incident
in the lens 3 of the scanner having been scattered at a greater
angle and therefore partly counteracting the attenuation effect of
the pixel field 5. These conditions are elucidated, once again, on
the basis of the arrow, drawn with dashed lines, in FIG. 2(a). As
can be identified from FIG. 2(b), no stray light that is incident
in the lens 3 at this angle arises in the WLCD since the pixels
that would produce this stray light are not illuminated in the WLCD
on account of the smaller illumination field. If the intent is now
to determine the effective attenuations or intensities for the WLCD
and the scanner, as produced by the pixel field, it is sufficient
to integrate the kernel over the respective tool-specific
integration limits of the illumination directions. Expressed
differently, the integration limits of the integral contain the
necessary information about the tool-specific extent of the
illumination field for the WLCD and the scanner.
[0047] In partial figure (a), FIG. 3 shows, in an exemplary manner,
a regular lines-and-spaces structure used for the CDrun method and,
in partial figure (b), it shows the diffraction image arising in
the pupil; the zero and the first order of diffraction are clearly
visible.
[0048] As already mentioned previously, a change of the CD within
the mask structure is reflected in a change in the ratios of the
intensities of the zero and the first order of diffraction of the
diffraction pattern.
[0049] FIG. 4 shows a plot of the ratio of the intensities of the
orders of diffraction in the pupil over the average CD values,
likewise established by use of the WLCD but established by use of
aerial image measurements; this calibration renders it possible to
assign a CD change to any change in the ratio of the
intensities.
[0050] The effects of writing a pixel field into the mask are once
again elucidated on the basis of FIG. 5. It is possible to
recognize that the effect of this process lies in a general
intensity attenuation of both orders of diffraction of the
diffraction pattern. However, the ratio of the two intensities
remains unchanged as both intensities are attenuated by the same
factor.
[0051] FIG. 6 illustrates the option of comparing the established
absolute intensities before the CDC process and after the CDC
process and of establishing a percentage or relative attenuation
therefrom.
[0052] Referring to FIG. 7, in some implementations, a system 100
is provided for correcting the critical dimension uniformity of a
substrate, such as a photomask for semiconductor lithography. A
substrate 107 (in this example, a photomask) may be placed on a
holder on a movable XYZ stage 109. The system 100 may include a
laser source, such as an ultra-short pulsed femtosecond laser 101,
in which a computer or a central computerized control unit 121 may
control the timing of pulses of the light source. The fundamental
frequency of the laser pulse may be multiplied to higher harmonics
by a harmonics generator 102, which may be also accompanied by a
variable attenuator 103, for controlling output energy.
[0053] An attenuated laser beam is directed into a beam delivery
system that is synchronized with laser pulses timing and with a
3-axis moving stage 109 by the central computerized control unit
121. Subsequently the beam is focused by a focusing optics (e.g.
main objective lens 106) into the substrate of the photomask 107,
to write pixels within the substrate. The same objective lens 106
may be used for an in-situ machine vision system, which acts as a
microscope with high magnification. This microscope can be used for
measuring pixel size and shape as well as monitoring of the laser
breakdown process inside the substrate. It may also be used for
accurately positioning the pixels at designated locations within
the substrate. The photomask 107 is illuminated by a light source
113 via a light guide 110, and a variable aperture stop 111. The
areas, shapes, and positions of the pixels are chosen to match the
numerical aperture and illumination mode of the lithographic
process, for which eventually the photomask 107 is used. Light may
be collimated via a condenser lens onto the patterned layer 108 of
the photomask 107.
[0054] An image is eventually formed via objective 106,
beam-splitter 104, and a tube-lens 118, which directs the light to
a charge coupled device (CCD) camera 119. An image may be grabbed
by a continuous frame-grabber 120, and processed at the central
computerized control unit 121. A UV source 116 provides UV light
that is directed at the photomask 107, and a detection system 117
measures and reads the UV irradiation attenuation level of each
area having pixels inside the substrate 107. An additional imaging
system 115 (using light source 114) with low magnification, may be
used for navigation across the photomask 107 and for determining
coordinates of alignment marks, so CD variation tables which may
contain XY alignment coordinates may be loaded to the computer 121,
and match the laser patterning process.
[0055] In some implementations, the computer 121 can perform the
processing of data described above, including calculations of
various coefficients and parameters. The computer 121 can receive
data about the angle-dependent scattering behavior of the pixel
fields from a storage device, in which the data may be obtained
from measurements of pixel fields using the ellipsometer. The
computer 121 simulates the effects of the scanner and the effects
of the pixels to determine the transfer coefficient, which is then
used in the process of verifying whether the photomask has been
corrected as described above. The characteristics of the scattering
of the pixels are used in the simulation of the effects of the
pixels.
[0056] The computer 121 can be implemented using a system that
includes one or more processors and one or more computer-readable
media (e.g., ROM, DRAM, SRAM, SDRAM, hard disk, optical disk, and
flash memory). The one or more processors can perform various
computations described above. The computations can also be
implemented using application-specific integrated circuits (ASICs).
The term "computer-readable medium" refers to a medium that
participates in providing instructions to a processor for
execution, including without limitation, non-volatile media (e.g.,
optical or magnetic disks), and volatile media (e.g., memory) and
transmission media. Transmission media includes, without
limitation, coaxial cables, copper wire, fiber optics and free
space. The memory can include any type of memory, such as ROM,
DRAM, SRAM, SDRAM, and flash memory.
[0057] The features described above can be implemented
advantageously in one or more computer programs that are executable
on a programmable system including at least one programmable
processor coupled to receive data and instructions from, and to
transmit data and instructions to, a data storage system, at least
one input device, and at least one output device. A computer
program is a set of instructions that can be used, directly or
indirectly, in a computer to perform a certain activity or bring
about a certain result. A computer program can be written in any
form of programming language (e.g., C, Java), including compiled or
interpreted languages, and it can be deployed in any form,
including as a stand-alone program or as a module, component,
subroutine, a browser-based web application, or other unit suitable
for use in a computing environment.
[0058] Suitable processors for the execution of a program of
instructions include, e.g., general purpose microprocessors,
special purpose microprocessors, digital signal processors,
single-core or multi-core processors, of any kind of computer.
Generally, a processor will receive instructions and data from a
read-only memory or a random access memory or both. The essential
elements of a computer are a processor for executing instructions
and one or more memories for storing instructions and data.
Generally, a computer will also include, or be operatively coupled
to communicate with, one or more mass storage devices for storing
data files; such devices include magnetic disks, such as internal
hard disks and removable disks; magneto-optical disks; and optical
disks. Storage devices suitable for tangibly embodying computer
program instructions and data include all forms of non-volatile
memory, including by way of example semiconductor memory devices,
such as EPROM, EEPROM, and flash memory devices; magnetic disks
such as internal hard disks and removable disks; magneto-optical
disks; and CD-ROM, DVD-ROM, and Blu-ray BD-ROM disks. The processor
and the memory can be supplemented by, or incorporated in, ASICs
(application-specific integrated circuits).
[0059] While this specification contains many specific
implementation details, these should not be construed as
limitations on the scope of any inventions or of what may be
claimed, but rather as descriptions of features specific to
particular embodiments of particular inventions. Certain features
that are described in this specification in the context of separate
embodiments can also be implemented in combination in a single
embodiment. Conversely, various features that are described in the
context of a single embodiment can also be implemented in multiple
embodiments separately or in any suitable subcombination.
[0060] Thus, particular embodiments of the subject matter have been
described. Other embodiments are within the scope of the following
claims. In some cases, the actions recited in the claims can be
performed in a different order and still achieve desirable
results.
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